1. Field of the Invention
The present invention relates to an electric motor and, more particularly, to a drive system for the electric car.
2. Description of the Related Art
Some prior art electric cars utilize a brushless motor having a stator arranged around a rotor having a permanent magnet to produce the driving torque necessary to run the car.
In such motors, magnetic reactance fluctuates as the permanent magnet fixed on the rotor passes the stator pole gaps in the stator, so that a cogging torque (or slot ripple torque) composed of higher harmonics is superposed on the driving torque.
Specifically, the permanent magnet of the rotor is spaced from the inner circumference of the iron core of the stator to provide a constant rotational gap. The iron core of the stator is composed of an annular stator yoke, and a plurality of stator poles projecting radially inward from the stator yoke. As the rotor rotates, the magnetic flux through a given stator pole is moved to an adjacent stator pole. Since the adjoining stator poles are separated by a stator pole gap the movement of the magnetic flux is discontinuous. When the corner of the permanent magnet passes over the stator pole gap, the attraction and/or repulsion between the rotor and the stator poles pulsates to produce the cogging torque.
Because the motor operates over a wide range of angular velocities from zero or low angular velocity to a high angular velocity when mounted in the drive system of an electric car, vibration of the motor by the cogging torque may cause a resonance to occur at the intrinsic vibration resonant frequency of the suspension system of the car body and the motor. Therefore in one proposed prior art motor, the stator or rotor is skewed to prevent changes in the magnetic reactance when the corner of the permanent magnet passes over the stator pole gap (as disclosed in Japanese Patent Laid-Open No. 8853/1989).
In another proposal, the permanent magnet is magnetized to produce a sine-wave distribution of magnetic flux. This sine-wave magnetic flux distribution is designed to prevent cogging torque.
In the prior art electric motor with the skewed stator or rotor, the permanent magnets or the stator poles have to be constructed to establish the predetermined skew angle. This construction makes wiring winding operations difficult to automate so that production cost is increased. Also when the stator is skewed, the working efficiency of the motor is seriously degraded.
In motors designed to have rotors with sine-wave magnetic flux distribution the proper construction of the permanent magnets is difficult. Also the efficiency of motors where the permanent magnets have a sine-wave magnetic flux distribution is less than the efficiency of motors where the permanent magnets are magnetized with a square-wave magnetic flux distribution. Furthermore, the motors with a sine-wave permanent magnetic flux distribution have a lower maximum torque than motors with square-wave permanent magnetic flux distribution.